1. Field of the Invention
This invention is directed to a system for accurately controlling the flow rate of a drug from an implantable drug delivery device. One device of this type is the implantable infusion pump described in U.S. Pat. No. 3,731,681. This invention is an improvement over the system disclosed in U.S. Pat. No. 4,299,220 which relates to a compensating mechanism for an implantable pump. These devices are used to deliver drugs at a very slow rate over a long period of time prior to subcutaneous refill.
2 . Prior Art
U.S. Pat. No. 3,731,681 describes an implantable infusion pump which utilizes the vapor pressure of a two stage gas to maintain a constant pressure on a drug flowing through a capillary tube in order to maintain a constant flow rate. This technique of flow control while simple and reliable is sensitive to outside variables such as body temperature and atmospheric pressure. Because the temperature of the body in which the device is implanted remains relatively constant, the vapor pressure also stays constant. Thus, while the patient remains at one local ground level air pressure is also essentially constant. However, there are conditions in which both temperature and atmospheric pressure may significantly change. If, for example, the patient has a high fever and at the same time takes a ride in an airplane where the atmosphere pressure is much lower than at sea level the high vapor pressure inside the pump due to the fever and the low pressure outside the pump due to the high altitude will cause pump to flow at a rate about 25% higher than at standard temperature and at sea level. While the drug dosage can be adjusted by changing the concentration of drug in the pump this, never the less, is a serious inconvenience and hardship for the patient.
U.S. Pat. No. 4,299,220 defines an improved implantable pump system employing a regulator to compensate for variations in pressure and temperature and thereby insure a more accurate and uniform rate of drug delivery. The implantable flow regulator of the '220 patent employs a body having a shallow internal cavity and a flexible diaphragm in the body which divides the cavity into two chambers. An inlet is provided to each of the chambers. An outlet is provided leading from the second of the chambers and is centrally disposed in the wall of the cavity underlying the diaphragm. Thus, flexing of the diaphragm in one direction contacts an elastomeric sealing ring around the outlet and closes the fluid passage way. The inlet of the regulator body is adapted to be connected to the capillary flow line from a pressure actuated drug delivery device. The flow line includes at least one capillary restrictor upstream from the inlet to the second chamber. The outlet from the second chamber of the regulator body is adapted to be connected to a catheter flow line extending to the desired infusion cite within the patient.
The capillary restrictor is thus in series with the flow control valve which is formed by the outlet and the diaphragm. The normal pressure drop across the capillary is less than the minimum pressure drop across the entire system, that is the pressure difference between the drug chamber and the distal end of the catheter in the outside environment of the infusion site. The pressure drop across the capillary tube is sensed by the diaphragm. When the opposing forces on the diaphragm are stable, the diaphragm is stationary. If there is a change in these forces, the diaphragm deflects either to close the valve when the pressure difference is negative in the second chamber or to open the valve if the pressure difference is positive in the second chamber.